New Mexico Geological Society Annual Spring Meeting — Abstracts

Convective geothermal systems in the Rio Grande rift and adjacent areas occur over structurally high terrane that is adjacent to regional drainage (Witcher, 1988), Three primary, but local, types of geologic settings are observed within the regional framework. AI deep reservoir or circulation framework for the Rincon-type setting is probably provided by fractured basement at convergent structural boundaries of Laramide uplifts. The geothermal systems at Rincon and San Diego Mountain are examples of the Rincon-type setting. Closespaced
faulting associated with mid-Tertiary extensional terranes and silicic cauldron complexes provides widely-distributed fracture permeability for the Tortugas-type geothermal setting. The Las Cruces East Mesa system is an example of the Tortugas-type setting. In other areas, a single fault zone may provide the deep circulation framework. Gila Hot Springs is a probable example of the fault-plane geothermal setting.

Geohydrologic windows allow discharge across confining aquitards (Witcher, 1988 and Barroll and Reiter, 1990). Three categories of geohydrologic windows are apparent in the primary geologic settings. For some systems, the shallow reservoir and the discharge window may be synonymous. A fractured intrusive provides small and limited discharge window for the Radium Springs geothermal system. The discharge window at Tortugas Mountain on the Las Cruces East Mesa geothermal system is relatively extensive and results from erosional stripping of aquitards from a fractured Tertiary horst block. At Rincon, a small discharge window is formed by a Tertiary fault that has tectonically eliminated a major aquitard and juxtaposed lithology with good potential for fault-zone fracture permeability.

Quaternary faults are associated with many, but not all, systems in the area. Recurrent Quaternary faulting probably enhances subsurface fracture permeability, especially with higher temperature systems that will tend to seal off permeability with mineralization and alteration. Young faults that merge with older Laramide and Tertiary structures are probably very important in reopening older, fractured volumes of rock.

Depending on the geologic setting, the deep reservoir storage capacity will vary. Maximum storage is found in the Tortugas and Rincon settings, while minimum storage occurs with the fault-plane setting. Given the same regional heat flow, recharge potential, and deep reservoir characteristics, the average reservoir temperature will vary inversely with the effective area of the discharge geohydrologic window. The effective area of the discharge window is related to the thickness and lateral extent of the window and the
magnitude and type of window permeability.